A joining method using a solder (solder paste) is widely used in mounting an electronic part.
In the Sn—Pb solder previously widely used, a bonding method with temperature hierarchy is widely applied, in which soldering is performed at a temperature of 330 to 350° C. by using Pb-rich Pb-5Sn (melting point: 314 to 310° C.) or Pb-10Sn (melting point: 302 to 275° C.) as high temperature solder, for example, and then soldering is performed at a temperature equal to or lower than the melting point of the high temperature solder by using eutectic Sn-37Pb (melting point: 183° C.) of low temperature solder, for example, and thereby, a connection by soldering is performed without melting the high temperature solder used in previous soldering.
Such a bonding method with temperature hierarchy is applied to a semiconductor device of a type in which a chip is die bonded or a semiconductor device which is subjected to flip-chip bonding, and it is an important technique used for the case where connection by soldering is performed in a semiconductor device, and then the semiconductor device itself is connected to a substrate by soldering.
As a solder paste used for this use, for example, a solder paste including a mixture of (a) second metals such as Cu, Al, Au and Ag, or a second metal (or alloy) ball made of a high melting point alloy containing the second metal and (b) a first metal ball made of Sn or In is proposed (see Patent Document 1).
Further, in Patent Document 1, a joining method using a solder paste and a method for manufacturing electronic equipment are disclosed.
When soldering is performed by using the solder paste described in Patent Document 1, as shown schematically in FIG. 2(a), a solder paste including low melting point metal (for example, Sn) balls 51, high melting point metal (for example, Cu) balls 52 and a flux 53 reacts by heating, and after soldering, as shown in FIG. 2(b), a plurality of high melting point metal balls 52 are connected to one another with an intermetallic compound 54, which a low melting point metal derived from the low melting point metal ball forms with a high melting point metal derived from the high melting point metal ball, interposed therebetween, and objects to be joined are connected (soldered) to each other by this connected body.
However, in the case of the joining method of Patent Document 1 in which a solder paste is used, an intermetallic compound of the high melting point metal (for example, Cu) and the low melting point metal (for example, Sn) is produced by heating the solder paste in a soldering step, and when a combination of Cu (high melting point metal) and Sn (low melting point metal) is employed, the low melting point metal, Sn, remains since its diffusing rate is low. When Sn remains, joint strength under an elevated temperature is significantly deteriorated, and there may be cases where a product to be joined cannot be used depending on the type thereof. Further, there is a possibility that Sn remaining after the step of soldering may be melted and flown out in a subsequent step of soldering, and there is a problem that this solder is low in reliability as high temperature solder to be used for a bonding method with temperature hierarchy.
That is, for example, in a manufacturing process of a semiconductor device, when a semiconductor device is manufactured after undergoing a step of soldering and then the semiconductor device is to be mounted on a substrate by a reflow soldering method, there is a possibility that Sn remaining after the step of soldering in the manufacturing process of a semiconductor device is melted and flown out in a step of reflow soldering.
Further, it is necessary to heat the solder paste at a high temperature for a long time in the soldering step in order to convert the low melting point metal completely to the intermetallic compound so that no Sn may remain, but actually this heating is practically impossible in consideration of a balance with productivity.
Moreover, when the solder paste described in Patent Document 1 is used, as shown in FIG. 3, a laminer intermetallic compound 64 such as Cu3Sn or Cu6Sn5 is formed at an interface between objects to be joined 61, 62 after reflow and a joining material (solder) 63. If such a laminar intermetallic compound 64 is formed, joint strength at the interface is deteriorated through production of cracks or the like because stress is concentrated at the interface.    Patent Document 1: Japanese Patent Laid-open Publication No. 2002-254194